1. Field of the Invention.
This invention relates to improved unleaded gasoline compositions. More particularly, the invention relates to the incorporation of molybdenum(VI) compounds into an unleaded gasoline for the purpose of reducing elevated steady state octane requirement and suppressing octane requirement increase in spark ignition internal combustion engines.
2. Description of the Prior Art.
A great number of gasoline additives, including antiknock agents, deposit reducing agents, demulsifiers, etc., have been developed in recent years. A commercially important antiknock agent, tetraalkyl lead, has, until recently, been universally used to prevent engine knock by increasing the octane number of gasoline. However, with increased sensitivity to lead in the environment and with the use of catalytic converters that can be poisoned by lead, broad restrictions have been placed on the use of lead in gasoline. In the absence of lead, greater amounts of expensive, high octane blending stock must be used to produce gasoline having sufficient octane for current production automobiles.
Along with the increased cost of production of unleaded gasoline, a particularly harmful problem has arisen. Engines operating on unleaded gasoline commonly experience increasing incidence and severity of knock as they age. A new or "clean" engine can operate efficiently and without knock using a gasoline having a research octane number of about 85. The same engine with about 8,000-12,000 accumulated miles can often require a gasoline having an octane number about 95-100 or higher. This increase in the octane number required to prevent knock is called octane requirement increase (ORI).
ORI is believed to be one result of thermally insulating combustion chamber deposits formed from gasoline contaminants and from the incomplete combustion of gasoline and lubricating oil. Initially the rate of deposit formation is substantially greater than the rate of disintegration, and the deposits rapidly build. As the deposits thicken, the rate of disintegration approaches and eventually equals the rate of formation. At this point, the deposits reach a steady state thickness. Knock in the engine appears to increase in incidence and severity as the deposit builds and reaches a constant or steady state elevated rate of incidence and severity corresponding to the steady state thickness of the deposit. At this point the engine commonly has an elevated steady state octane requirement which can be 2 to 15 research octane numbers greater than the octane requirement when new.
While we do not wish to be limited to a theory of ORI, we believe that the combustion chamber deposits have the substantial ability to prevent transfer of thermal energy from the combustion chamber into engine coolant, causing accumulation of thermal energy in the deposits and in the combustion chamber. When a spark ignites the air/fuel mixture in the combustion chamber, a flame front is initiated and combustion rapidly and smoothly progresses from the spark plug to the "end-gas region" opposite to the spark plug. The high pressure flame front rapidly compresses the unburned air/fuel mixture which is at a relatively lower pressure in the end-gas region as the front progresses through the chamber. Commonly, the combustion progresses through the combustion chamber, and knock is not heard. However, if the temperature of the combustion chamber and the air/fuel mixture has been substantially increased by the insulating properties of the deposits, the rapid compression of the air/fuel mixture in the end-gas region causes an immediate autodetonation which is different than normal progressive combustion. This autodetonation causes the "knocking" or "pinging" sound, can reduce operating efficiency and can cause engine damage. See J. D. Benson, "Some Factors Which Affect Octane Requirement Increase," SAE Paper 750933, Detroit, Mich., October 1975, for a detailed treatment of ORI.
ORI can readily be remedied with leaded gasoline by increasing the lead concentration. In unleaded fuels, a greater amount of high octane blending stock must be used to increase the octane. However, high octane blending stock commonly contains aromatic constituents that are more likely to leave thermally insulating deposits and increase ORI.
With the suppression of ORI, more gasoline with a lower octane number could be produced per barrel of crude oil at lower cost. Since the production of high octane gasoline consumes more energy than the production of lower octane gasoline, refining operations would become more energy efficient. Further, in the absence of ORI, engine manufacturers could build more fuel efficient engines by increasing compression and adjusting spark timing. Such engines would perform satisfactorily with a fuel having the same or lower octane as is currently available.
In this application, octane requirement increase shall mean the gradual increase in octane requirement observed as an engine ages. Elevated steady state octane requirement shall mean the octane requirement of an engine with combustion chamber deposits that have reached a steady state both in thickness and in resistance to thermal energy flow.
The incorporation of certain specific molybdenum compounds into gasoline has been suggested for the purpose of providing a composition having improved lubricating and antiwear properties. U.S. Pat. Nos. 4,164,473; 4,176,073; and 4,176,074 disclose the use of molybdenum complexes of hydroxy amines, molybdenum complexes of lactone oxazoline dispersants, and molybdenum complexes of oxazoline dispersants respectively for this purpose. Similarly, U.S. Pat. Nos. 4,192,757 and 4,201,683 disclose, for this purpose, the use of molybdenum complexes which are obtained by reaction of a hydrocarbyl substituted thio-bis-phenol with a molybdenum compound in the presence of an amine in an alkyl substituted phenol or alkanol solvent, respectively. In addition, U.S. Pat. No. 3,994,697 teaches that a solid pellet comprising various metals in combination with molybdenum disulfide can be placed in the fuel reservoir of an internal combustion engine where it slowly disintegrates to produce extremely minute particles which are dispersed in the fuel and are delivered to the fuel-contacting parts of the engine to deposit a lubricant film thereon.
U.S. Pat. Nos. 3,615,293 and 3,755,195 disclose the incorporation of various organic molybdenum compounds into a gasoline fuel which contains an organomanganese antiknock agent. These patents teach that the molybdenum compounds are effective in reducing spark plug fouling by gasoline fuels which contain organomanganese antiknock agents. It is further disclosed that suitable organic molybdenum compounds include molybdenum salts and chelates. However, these references fail to suggest the incorporation of a molybdenum compound into gasoline for any purpose in the absence of an organomanganese antiknock agent.
U.S. Pat. No. 3,317,571 discloses the preparation of organomolybdenum compounds wherein one or more molecules containing an amide or thioamide linkage are bonded to the molybdenum atom through a sulfur or oxygen linkage and which is stabilized by additional covalent bonding to a plurality of carbonyl groups. This patent discloses that such compounds can be used in gasoline, either alone or in combination with lead alkyls, as antiknock agents. It fails, however, to either teach or suggest the use of a molybdenum(VI) compound for any purpose.
U.S. Pat. No. 3,272,606 discloses that small amounts of a covalent molybdenum polycarbonyl compound can be used in gasoline in combination with an organolead antiknock agent to enhance the antiknock properties of the organolead antiknock agent. However, this reference fails to suggest the incorporation of a molybdenum compound into gasoline for any purpose in the absence of an organolead antiknock agent.
U.S. Pat. No. 2,086,775 is directed to the incorporation of various organometallic compounds into a liquid fuel for an internal combustion engine. Suitable organometallic compounds are those of cobalt, nickel, manganese, iron, copper, uranium, molybdenum, vanadium, zirconium, beryllium, platinum, palladium, thorium, chromium, aluminum, and the rare earth metals. In addition, the liquid fuel may also contain an organolead antiknock agent. It is disclosed that use in an internal combustion engine of a fuel containing small amounts of these organometallic compounds results in the formation of a catalytic deposit within the combustion chambers of the engine which is effective for the elimination of knock and the improvement of combustion within the engine. However, this patent offers no guidance for selecting a molybdenum(VI) compound for use in a gasoline composition which is substantially free of other metals for the purpose of suppressing octane requirement increase and reducing elevated steady state octane requirement in spark ignition internal combustion engines.
U.S. Pat. No. 3,155,620 is directed to the use of cycloheptatriene transition metal coordination compounds of the Group VIB metals as additives for liquid hydrocarbon compositions. This patent teaches that such additives can be used to increase the octane of liquid fuels and to provide improved lubricating properties when incorporated into lubricating oil compositions. It is further disclosed that these coordination compounds can be used in combination with antiknock agents such as organolead compounds. However, this patent fails to either teach or suggest the use of a molybdenum(VI) compound for any purpose.
U.S. Pat. No. 3,440,028 is directed to the incorporation of a metal halide hydrocarbyl orthophosphate additive into leaded gasoline compositions for the purpose of suppressing the tendency of the lead to increase undesirable surface ignition within the combustion chambers of an engine. The metal of the additive can be selected from the group consisting of manganese and metals of Groups IB, IIA, IIB, IVA, VIB and VIII of the Periodic Table. Similarly, U.S. Pat. No. 3,240,576 discloses that the addition to leaded gasoline of a gasoline soluble organomolybdenum compound will provide a substantial reduction of surface ignition in the combustion chambers of a spark ignition internal combustion engine. These patents do not, however, suggest the addition of a molybdenum compound to unleaded gasoline for any purpose.
U.S. Pat. No. 3,401,184 is directed to a method for the preparation of metal organo orthophosphates wherein the metal can be selected from Groups II, IV, VI and VIII of the Periodic Table. It is disclosed that these compounds have utility as gasoline additives and that when so used they impart rust inhibition, surface ignition suppression, carburetor detergency, carburetor icing alleviation and reduction in octane requirement increase to the gasoline composition. In addition, U.S. Pat. No. 3,282,838 discloses the use of amine salts of chromic or molybdic acid as corrosion inhibitors in petroleum hydrocarbons such as gasoline. These amine salts contain either a chromic or molybdic ion of +6 valence and are used at a concentration level between about 0.005 and 5 weight percent.
Finally, U.S. Pat. No. 3,003,859 discloses the incorporation into a liquid hydrocarbon, such as gasoline, of about 0.005 to about 5 percent by weight of a metal-organic material which is obtained by heating a normally-solid metallic chelate compound formed from a beta-keto ester to a temperature above its melting point. It is further disclosed that these metal chelates can be formed from metallic elements of the Periodic Table comprising the Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIB, VIIB and VIII including the lanthanide and actinide series of rare earth elements. This patent, however, fails to offer any guidance for selecting a molybdenum(VI) compound for the purpose of suppressing octane requirement increase and reducing elevated steady state octane requirement in spark ignition internal combustion engines.